1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus having a development voltage controlling unit.
2. Description of the Related Art
FIG. 1 schematically shows a structure of a conventional image forming apparatus. Referring to FIG. 1, the conventional image forming apparatus includes a photosensitive drum 10, a laser scanning unit (LSU) 11 which forms an electrostatic latent image by radiating light onto the photosensitive drum 10, developing rollers 13a, 13b, 13c, and 13d which develop the electrostatic latent image by supplying a developing agent to the photosensitive drum 10, a first transfer roller 17 which transfers the image developed by the developing rollers 13a, 13b, 13c, and 13d onto an intermediate transfer belt 15, a second transfer roller 19 which transfers the image of the intermediate transfer belt 15 onto a sheet of paper S, and a fusing roller 21 which fixes the image on the paper S by heating and pressurizing the image. Developing agents having four colors, such as yellow (Y), cyan (C), magenta (M), and black (K), are supplied to the developing rollers 13a, 13b, 13c, and 13d, respectively.
Here, a gap ring 12 is inserted around the developing rollers 13a, 13b, 13c, and 13d, such that surfaces of the developing rollers 13a, 13b, 13c, and 13d do not contact the photosensitive drum 10 and are spaced-apart by a gap formed between the surfaces of the developing rollers 13a, 13b, 13c, and 13d and the photosensitive drum 10. The conventional image forming apparatus includes a turret 14, which rotates each of the developing rollers 13a, 13b, 13c, and 13d in response to an input signal.
In the conventional image forming apparatus, in order to print the image on the paper S, the photosensitive drum 10 is charged by a charger (not shown), the light is radiated onto the photosensitive drum 10 by the LSU 11 to reduce a predetermined electric potential of the photosensitive drum 10, thereby forming the electrostatic latent image, while the developing rollers 13a, 13b, 13c, and 13d sequentially correspond to the photosensitive drum 10, non-contact development is performed on the photosensitive drum 10 by the gap formed by the gap ring 12, thereby developing the electrostatic latent image. The developed image is transferred onto the intermediate transfer belt 15 by the first transfer roller 17, and the image on the intermediate transfer belt 15 is transferred onto the paper S by the second transfer roller 19. The image on the transferred paper S is fixed on the paper S by the fusing unit 21.
In the conventional image forming apparatus, in a development operation, a direct current (DC) development voltage and an alternating current (AC) development voltage overlap to be applied to the developing rollers 13a, 13b, 13c, and 13d, thereby forming the developed image on a surface of the photosensitive drum 10. However, when the developing rollers 13a, 13b, 13c, and 13d are used for a long time, the gap ring 12 is worn down, such that the gap between the surface of the photosensitive drum 10 and the surfaces of the developing rollers 13a, 13b, 13c, and 13d is reduced. Thus, a development electric field formed on the photosensitive drum 10 and the surfaces of the developing rollers 13a, 13b, 13c, and 13d is increased. In this case, the overlapped development voltage of the DC and AC development voltages is excessively applied to the developing rollers 13a, 13b, 13c, and 13d. Thus, a leakage of the development voltage occurs, and spots occur in the image, or image concentration is excessively increased.